A function required for an optical system used in an image capture apparatus is a function of gathering light incident from a subject by bending the light to thereby form an image of the subject on an image plane. Examples of an optical element having such an action of bending light include a lens that bends light by utilizing a difference in refractive indexes and a reflector that bends light through reflection.
As for the lens, light passes through an inside thereof, so it is required to use a material having sufficiently large transmittance with respect to a desired wavelength range. Further, a material is limited to an expensive material depending on a specific wavelength range such as ultraviolet rays or infrared rays, which leads to a problem in terms of cost.
In addition, there is chromatic aberration ascribable to variances in a magnitude of a refractive index of the material of the lens that generally occur depending on a wavelength of light, so in order to obtain constant image formation performance over a wide wavelength range, it is required to perform so-called achromatization that is a complicated correction in which, for instance, two or more lenses having different refractive index changes with respect to a wavelength are combined with each other.
As for the reflector, there is no limitation on a material thereof as long as a reflection surface can be coated with a reflection material having sufficient performance, so it is possible to obtain an inexpensive optical system with respect to every wavelength range. Also, a reflection action does not depend on the wavelength of light, so it is possible to obtain an optical system having no chromatic aberration over a wide wavelength band with ease.
In a reflection-type optical system, however, incident ray to a reflection surface and reflection light beams thereof appear on the same side with respect to the reflection surface, so a reflector of a next surface is also positioned on the same side as the incident ray. Therefore, a so-called “eclipse” that is a phenomenon in which the reflector of the next surface obscures the incident ray readily occurs. When the eclipse occurs, a light amount of the incident ray decreases, so it becomes impossible to obtain a bright optical system.
To prevent the eclipse from occurring, there is a method of giving an angle between a light beam incident direction and a light beam outgoing direction by using, for instance, incident ray inclined with respect to an axis of the optical system. As a matter of course, the reflector of the next surface is arranged in the light beam outgoing direction, so by changing a direction by giving an angle between the light beam incident direction and the light beam outgoing direction, it is possible to prevent overlapping of the reflector of the next surface with the incident ray, to thereby prevent the eclipse.
For instance, FIG. 13 is a cross-sectional view showing a conventional imaging optical system of a reflection type (see Non-patent Document 1 and Patent Document 1). A coordinate system in the figure is expressed with a right-handed coordinate system in which a righthand direction in the figure is set as a “+z direction”, an upward direction in the figure is set as a “+y direction”, and an upward direction vertical to a paper plane is set as a “+x direction”.
The conventional imaging optical system shown in FIG. 13 uses light beams having predetermined inclinations in a y direction with respect to an axis 11 of the optical system as incident ray 12 incident to the optical system and bends the incident ray 12 in a zigzag manner on a plane, thereby enabling an arrangement in which the light beams do not overlap with a first reflector 1 having a convex spherical shape, a second reflector 2 having a concave rotation oval surface shape, a third reflector 3 having a concave spherical shape, an aperture diaphragm 10, and an image plane 4 in the y direction. As a result, the optical system realizes an angle of view of 30°×20° at a brightness of F/4.
Non-patent Document 1: Kenneth L. Hallam et al., “An all-reflective wide-angle flat-field telescope for space”, Instrumentation in astronomy V; Proceedings of the Fifth Meeting, 1983
Patent Document 1: U.S. Pat. No. 4,598,981 (WIDE-ANGLE FLAT FIELD TELESCOPE)